1. Field of the Invention
This invention relates to a copper foil structure with enhanced flexibility for use in flexible printed circuit boards, and more specifically to a deposition method of forming a thermally stable layer overlying the matte side of a super-HTE (high temperature elongation) electrolytic copper foil.
2. Description of the Related Art
Flexible printed circuit boards are widely used for repetitive flex motion devices from office appliances such as printers and hard disk drives to telecommunication gadgets such as mobile phones, PDAs . . . etc., because of the superb fatigue performance thereof.
As copper is a conductive element, copper foils laminated on the FPC board must be conformal to fatigue. Two kinds of copper foil, electrolytic foil and rolled foil can be categorized by their respective manufacturing processes. Rolled and annealed copper foils, because of their superior fatigue performance, have been traditionally used in most FPC boards. High costs, direction-related properties and dimensional limitations (60 cm max in width), however, have impeded further development. On the other hand, conventional electrolytic copper foils made from high chloride concentration (>30 ppm) electrolyte generally have low fatigue ductility (20–50%). Accordingly, electrolytic copper foils can only be used for single bend usage items like dashboards or relatively large radius bends. However, the situation is currently changing now. A newly developed electrolytic copper foil has recently been realized and offers great improvement in high temperature elongation performance (>15%). This new copper foil is gradually gaining acceptance in the FPC field, as evidenced by its filing in the material code IPC-4562 (May, 2000), and also from the fact that other commercial rolled copper foil like M-BSH had been retired.
The structural characteristics of the electrolytic copper foil are very much dependent on processing conditions. Traditionally, the side directly contacting a cathode is known as the “shiny side”, due to the fine grain size and a bright, smooth appearance thereof. Whereas the opposite side close to electrolyte is known as the “matte side”, due to the rougher surface which is easily concurrent with the columnar grain structure contributed by the high chloride content. The matte side is often pink in color.
The flexible characteristics of copper foil are critical to their adaptation in FPC boards and is tested and reported on by the strain controlled fatigue tests. Unfortunately, there is still no universally accepted method up to now. The two most prevalent test methods are the bell-flex test specified in IPC-TM-650 and MIT folding endurance test filed as JIS-P-8115 or ASTM-D2176, popular in USA and Japan respectively. According to IPC-TM-650, the flex cycle number of the tested copper foil specimen must be limited in between 30–500 times by adjusting the mandrel diameter, and a Df (fatigue ductility) value can then be numerically analyzed. A larger Df value means a better flex performance for the tested specimen. On the other hand, with a fixed folding rate (175 cpm) and suitable assignment of load (e.g. 500 g) and radius of curvature of the bend faces (e.g. R=0.8 mm), the copper foil will be bend cyclically and then the bent cycles can be reached without failure will be recorded as Nf and directly used as performance index for the MIT test. As with the Df, a larger Nf also means a better folding performance.
Due to different methodologies and parameters used, the two different fatigue tests mentioned above can often result in conflicting conclusions for the same tested copper foil sample. Let's take the annealing effect on commercial SHTE electrolytic copper foil as an example. According to the inventors' discoveries, a 180° C.-60 min annealing treatment can indeed show advantageous effect on Dfs of copper foil as expected, but, on the contrary, will result in a detrimental effect on MIT-Nf. This inconsistency of annealing effect has puzzled the local FCCL and FPCB industries, which use the MIT test, and made the acceptance of electrolytic copper foil more difficult. Therefore, for making diversification more easy, all newly developed copper foil must face two challenges, that is (1) a better and more stable flex performance, and (2) a consistent advantageous annealing effect on flex performance must be guaranteed for both fatigue tests.